Subsea Xmas tree production valves are controlled by actuators. Traditionally, the actuators have been hydraulic, which is considered reliable and rather safe. However, a shift towards electrical actuators is under development in the industry. This is mainly due to longer step outs of subsea installations. Longer step outs result in slower response time using hydraulic systems, as well as increased cost of hydraulic umbilicals. An electric actuator comprises an electric motor coupled mechanical, typical with a mechanical transmission, to the valve. The electrical actuator is coupled or integrated with an electrical power and control system.
Electric actuators are however somewhat less reliable than hydraulic actuators, which is solved by adding redundancy, but in the case of leakage of contaminating fluid into the actuator the control of the actuator and thereby the valve is lost. Lost production for a period of time can be very expensive, but worse, if the leakage continues into the electrical control system a severe safety problem can be the result. A fluid leakage path from the valve to sea can be arranged in order to avoid leakage into the control system if the seal between the valve and actuator fails, but the leakage path into the control system still exists, as well as a pollution problem.
In subsea well operations, bore pressure of more than 690 bars is common. A typical state of the art actuator and valve assembly design is to have a valve body with a valve element, the valve element, such as a gate element of a gate valve, is connected to a valve stem. A dynamic seal prevents leakage of well fluid along the valve stem toward the actuator. The actuator is electrically operated and it includes an electric motor which via a transmission is connected to the valve stem. Leakage over the dynamic seal, along the stem or around the seal, will either flow into the actuator or to sea via a leakage path.
The dynamic seals are subject to wear, particularly for valves subject to frequent operation, such as anti surge valves of compressors. Number of open/close cycles, pressure, temperature and speed of movement are limiting parameters with regards to performance. Anti surge valves must have a very fast response in order to protect the compressor, the fast response excludes hydraulic actuators, and the maximum speed is typically limited by the dynamic seals around a stem or spindle.
A demand exists for valves being safer with respect to leakage and loss of control, the objective of the invention is to provide a subsea valve being favorable in this respect.
The closest prior art appears to be the international patent publication WO 2007/149795 A2, which relates to a wellbore flapper valve with an actuator with linear magnetic coupling through a production tubing. The actuator parts outside the production tubing are not protected in a sealed, clean, restricted, small volume and the flapper actuator parts inside the production tubing occupies a substantial part of the production flow bore area.
Another relevant prior art publication is patent application US 2009/0293957 A1 (Vetco), however, the illustrated and described device still has a valve stem with a dynamic seal, and there is no teaching to the person skilled in the art on how to solve the underlying problem of the present invention.
Other patents or applications relate to magnetic gears, such as US 2011/0057456 A1, U.S. Pat. No. 7,487,829 B2, US 2011/0037333, US 2007/215343 A1 and US 2007/125578 A1, relating to magnetic gears allowing gearing between force-torque and stroke-rotation, useful in drive mechanisms. EP 2021667 (Aker Subsea) relates to an electrically operated hydraulic valve with a magnetic coupling. None of publications referred to above, address the underlying problems of the present invention loss of control and pollution.